This invention relates to wavelength division multiplexed (WDM) coherent optical communication, which is alternatively referred to either as frequency division multiplexed (FDM) coherent optical communication or briefly as wavelength division multiplexed optical communication.
The wavelength division multiplexed optical communication can deal with a great number of channels and is effective for use in a large-capacity trunk-line optical communication system. In a transmitter of a wavelength division multiplexed optical communication network, signal beams of a plurality of channels of different wavelengths are wavelength division multiplexed into a multiplexed signal beam for transmission through an optical fiber. In a receiver which is typically an optical heterodyne receiver, the multiplexed signal beam is received through the optical fiber by using a local optical beam of a controllable wavelength which is adjusted so as to produce a data carrying intermediate frequency signal in response to one of the signal beams of one of the channels that should be received as a selected channel.
In the transmitter, laser diodes are used in general as optical beam sources for generating optical beams of the respective channels. It is known that each laser diode emits the optical beam of a source wavelength which varies in a wide wavelength range dependent on a bias current supplied to the laser diode and its ambient temperature. The controllable wavelength is therefore swept as a swept wavelength in a sweep range during start of reception of the multiplexed signal beam and subsequently stabilized by automatic frequency control (AFC) at an optimum wavelength after the data carrying intermediate frequency signal is obtained in response to one of the signal beams that is of the selected channel.
The sweep range must be sufficiently wide to cover the wavelength range. It may therefore happen that the controllable wavelength is stabilized at a wrong wavelength rather than at a correct wavelength of providing the data carrying intermediate frequency signal in response to the signal beam of the selected channel, particularly when the channels are spaced apart by a narrow wavelength or frequency separation. As a consequence, it is indispensable to use a channel discrimination method of discriminating between the channels, namely, the correct wavelength from wrong wavelengths, in order to insure stabilization of the controllable wavelength at the correct wavelength. In a conventional channel discrimination method, channel discrimination bits of a set are time division multiplexed on the signal beam of each channel.
It should be noted in connection with such an optical communication network that the multiplexed signal beam has a polarization state which is unavoidably subjected to fluctuations while the multiplexed signal beam is transmitted from the transmitter to the receiver through the optical fiber. It is known in the art that polarization control is effective in removing adverse effects which would otherwise result from the fluctuations. On carrying out the polarization control, a polarization control unit is used in the receiver to control the polarization state of a predetermined one of the multiplexed signal beam and the local optical beam so that the polarization state of one of the multiplexed signal and the local optical beams becomes coincident with that of the other of the multiplexed signal and the local optical beams. Thereafter, the multiplexed signal and the local optical beams are combined together into a combined beam for detection of the data carrying intermediate frequency signal.
More particularly, the polarization control unit is controlled so that the data carrying intermediate frequency signal may always have a maximum power level. Consequently, the polarization control is not carried out while the receiver is not supplied with the multiplexed signal beam. This may result in a disadvantage such that the data carrying intermediate frequency signal can not be obtained either when the multiplexed signal beam is first supplied to the receiver or when supply of the multiplexed signal beam is once suspended and then restarted.
In order to get rid of the disadvantage, an optical heterodyne homodyne detection apparatus is revealed in U.S. Pat. No. 4,903,342 which is issued to Yamazaki, the present inventor. According to the Yamazaki patent, the polarization control unit is subjected to scramble control, namely, is scramble controlled during start of sweep of the controllable wavelength to vary the polarization state of the predetermined one of the multiplexed signal and the local optical beams in a scrambled fashion so as to insure production of the data carrying intermediate frequency signal irrespective of the polarization state of the multiplexed signal beam.
The scramble control, however, results in an inconvenience when used in combination with the conventional channel discrimination method. The inconvenience is as follows. Inasmuch as it is impossible to demodulate the data carrying intermediate frequency signal into a reproduction of the channel discrimination bits during progress of the scramble control, the scramble control must temporarily be suspended when an intermediate frequency signal is obtained as a temporary intermediate frequency signal. This is in order to judge whether or not the selected channel is really indicated by temporary channel discrimination bits which are demodulated from the temporary intermediate frequency signal as a reproduction of the channel discrimination bits of a certain set. If the selected channel is not indicated by the temporary channel discrimination bits, the scramble control must be repeated until the selected channel is really indicated by such temporary channel discrimination bits. As a result, a long searching time interval is necessary to search for the selected channel before start of reception of the signal beam of the selected channel.